1. Field of the Invention
The present invention relates to a valve characteristic control apparatus for an internal combustion engine which controls valve characteristics such as valve timing (timing and phase for opening and closing a valve), valve opening time period (cam working angle), valve lift amount and the like.
2. Description of the Related Art
In order to achieve the optimal valve characteristic in accordance with the operating conditions of a vehicle engine, various mechanisms capable of changing the characteristics of a valve train have recently been developed and put to practical use. The valve characteristics include valve timing, valve opening time period, valve lift amount and the like. In order to change at least one of the aforementioned valve characteristics, there has been devised a valve train for changing rotational phase of a cam shaft relative to a crank shaft consecutively or in two steps (ON/OFF control), a valve train selectively using a plurality of cams having different profiles, a valve train for axially displacing a cam shaft having three-dimensional cams (solid cams) whose profile changes in the axial direction of the cam shaft, and the like. In an internal combustion engine equipped with a valve characteristic control apparatus as described above, the valve characteristic control is performed in accordance with the operating conditions of the engine in the light of intake efficiency and exhaust gas recirculation. A sufficient intake efficiency contributes to enhancement of engine power, and the exhaust gas recirculation brings about an improvement in the capacity for purging emission substances based on a reduction in NO.sub.x and an improvement in fuel consumption based on a reduction in pumping loss.
For example, Japanese Patent Application No. SHOU 55-87833 discloses a valve characteristic control apparatus for axially displacing a cam shaft having three-dimensional cams, which make it possible to change valve opening time period (cam working angle), valve lift amount and the like. Japanese Patent Application No. HEI 7-139327 discloses a valve characteristic control apparatus for consecutively changing valve timing by shifting a rotational phase of a cam shaft relative to a timing pulley. In this apparatus, the timing pulley is rotatably driven by a timing belt to which rotation of a crank shaft is transmitted. The timing pulley is connected via a ring gear to a cam shaft for driving valves. Formed on inner and outer peripheral surfaces of the ring gear are helical splines (longitudinally extending twisted grooves). A movable piston integrated with the ring gear is hydraulically driven to be displaced in the axial direction so that the cam shaft rotates relative to the timing pulley.
By combining the variable valve train employing three-dimensional cams with the variable valve train employing a ring gear, the valve characteristics including valve timing, valve lift amount and cam working angle can be controlled with a degree of freedom higher than in the conventional art. However, the apparatus with such a combination is bulky, complicated in structure and thus unsuitable for practical use.